Electric drive shafts on ships (including for example all-electric ships) conventionally include one or more electric drive motors which are each for driving a propelling unit (for example a propeller) and are each fed by way of a respective converter by an electrical power network on the ship (frequently also called the ship's drive). The electrical power network is in turn fed by one or more diesel generators. In this arrangement, the voltage of the electrical power network is of fixedly predetermined amplitude and frequency, for example having a medium voltage with a nominal voltage of 6.6 kV at a nominal frequency of 60 Hz. Where appropriate, a transformer is additionally connected between the converter and the power network. The converters convert the power network voltage (stepped down where necessary) to a voltage required to operate the drive motors, which has a different amplitude and frequency from the power network voltage.
Low-voltage consumers on board a ship (such as the navigation equipment and control gear, public-address system, lighting) are supplied by a separate onboard power network, which conventionally has a nominal voltage of 400 V and a nominal frequency of 50 Hz, or 440 V and 60 Hz. The onboard power network may be fed with electrical energy from its own onboard power network generators, independently of the ship's drive. As an alternative, the onboard power network may be supplied from the ship's drive, by way of an onboard power network converter. The onboard power network converter converts the voltage of the ship's drive to a voltage having the amplitude and frequency of the onboard power network.
A great advantage of this solution lies in the fact that disturbances to the ship's drive as a result of abrupt changes in load (for example if a propeller goes in and out of the water in heavy seas) can be avoided as a result of the converter if the latter is of appropriate size. Alongside many other advantages, however, these drive constructions have the disadvantage that they require a relatively large number of converters for the conversion of voltage in the ship's drive, and these entail a corresponding need for space, and costs.
A known electric drive solution which manages without converters of this kind consists in coupling the generators and the drive motors to one another with no converters connected in between. With a drive solution of this kind, one or more drive motors of variable speed are operated directly, without a converter connected in between, using the voltage generated by one or more generators of variable speed, this voltage being of variable amplitude and variable frequency.
Thus, the motors and thus the propelling units are controlled and/or regulated indirectly, by control and/or regulation of the internal combustion engines for driving the generators. In this case the drive motors are connected to the generators in a fixed electrical coupling, that is to say that a rotation of the generators brings about a corresponding proportional rotation of the electric drive motors. Thus, the function of a mechanical shaft is imitated using electrical machines. A drive solution of this kind is frequently called an “electric shaft”.
In this context it is also known to take off electrical energy from the electric shaft by way of an onboard power network converter, that is to say that an onboard power network converter converts the voltage generated by the generator (or generators), which is of variable amplitude and variable frequency, to a voltage of constant amplitude and constant frequency for an onboard power network.
Here, however, it is problematic that abrupt changes in load in the ship's drive directly cause disturbances, in the form of fluctuations in voltage and frequency, in the electric shaft and hence also in the onboard power network converter. This has the result on the one hand that complex regulation is required for the electric shaft, for stabilizing the speed (and hence the voltage and frequency) of the drive motors and the propelling unit. On the other hand, this may result in impermissible fluctuations in the onboard power network voltage. The onboard power network converter responds to this by performing a safety shut-off, which reduces the availability of the onboard power network, or else it has to be oversized such that it can compensate for the fluctuations in the ship's drive that are caused by the abrupt changes in load.